At their inception, guitar amplifiers were designed to merely amplify the sound of a guitar or other instrument and provide basic control over the tonality of the music being played. One such amplifier is shown in FIG. 1. It includes a first amplifier stage that buffers the incoming audio signal, a volume control, tone controls, a second amplifier stage, a third amplifier stage (which can include an effects mixer if reverberation or effects circuitry are utilized), and an output stage for providing the necessary driving power for the speakers.
In the early days of rock and roll, volume requirements exceeded power availability and, at these higher volume levels, the amplifiers produced an overdriven or distorted sound that became very popular. This type of guitar sound has now been widely used in popular music for over thirty years.
One problem that musicians encounter is achieving this overdriven sound at lower volume levels. Since this sound results from the amplifier being overdriven (i.e., at maximum volume), it cannot be created using conventional amplifiers at lower volume levels. Often, this makes it difficult to provide both the desired tonality for the music being played and the appropriate listening level for the particular setting in which the music is being played. Thus, for instance, an amplifier may produce one guitar sound at the limits of its power capabilities, but for a particular performance the volume requirements may be much less, and turning the amplifier down will yield a completely different tone.
Amplifier engineers have attacked this problem in different ways, but with limited results and, consequently, professional musicians usually own a variety of amplifiers in order to accommodate different situations.
A first approach to solving this problem was to add master volume control, as shown in the prior art amplifier circuit of FIG. 2. This essentially allowed the musician to turn up the pre-amplifier section to the desired overdrive level and regulate the amount of signal that went to the power amplifier section. Although better than the earlier amplifier designs, this design did not provide enough overdrive and it was not as sensitive to the guitarist's touch.
A second approach was a response to the first approach and its inherent shortcomings. As shown in the prior art amplifier circuit of FIG. 3, this approach involved adding more overdrive in the pre-amplifier section, by cascading voltage amplifiers before the master volume, in an effort to compensate for the loss in overdrive incurred by not including the output stage in the overdrive process. Although considered by most to be better than earlier designs, this approach still resulted in different sounds at different power levels and, as with the earlier designs, there was therefore an optimum power level at which the desired overdriven guitar sound was produced.
A third approach involved varying the power capacity of the output section to get it working harder. This was done by starving the outputs for voltage or impedance mismatch, or both. This feature was sometimes offered with the new high-gain pre-amplifiers and different versions of this approach have been state of the art since the early 1970's. However, the trend of owning a library of different amplifiers is still prevalent and still growing among most professional musicians.